Duplex Mismatch Question

hollow666 · 05-01-2009, 08:01 PM

So I'm tired of arguing with people about this issue but basically this is my understanding:

If a switchport is hardset with speed/duplex settings of 100/full and a workstation is set to autonegation or vice versa with the switch being set to auto and the workstation hardset, line speed will be detected properly based on electrical signaling but the duplex will default to half-duplex on the auto-negation link. This applies only to fastethernet and not gig ethernet.

Is this correct? Am I missing a piece here? I've had this happen to me multiple times and verified it but it seems like I'm constantly arguing with other networking people over this issue. I'm also noticing that alot of network guys don't seem to understand that the workstation itself has duplexing settings and just because the status tab on a windows machine says 100mbps that does not mean its full duplex. They seem to think that just because the switchport shows 100-full that the other end has to correspond.

networker050184 · 05-01-2009, 08:22 PM

Yes you are correct. If they give you crap just let them read this table.

rwwest7 · 05-01-2009, 08:42 PM

The hard coded side is at full-duplex, and the auto side is at half. The full-duplex side will act that way, causing all kinds of errors and collisions and a really slow connection.

Gigabit is auto only.

networker050184 · 05-01-2009, 09:27 PM

Quote:

Originally Posted by rwwest7

Gigabit is auto only.

Gigabit is not auto only. I have plenty of them hard coded at work.

pitviper · 05-01-2009, 09:58 PM

Auto settings are bad when one side is set to auto and the other is forced (especially on older 10/100 Cisco switches) - I've been burned a few times so now it's auto/auto for the non essential stuff and forced/forced for servers, printers, and everything Cisco.

hollow666 · 10:03 PM

Think he meant "full only" not auto? Gig will default to full duplex if autonegotiaton fails. You can still set gig to half-duplex manually though

rwwest7 · 05-02-2009, 12:05 AM

Quote:

Originally Posted by networker050184

Gigabit is not auto only. I have plenty of them hard coded at work.

"Autonegotiation is a requirement for using 1000BASE-T[4] according to the standard. At least clock source has to be negotiated, as one has to be Master and the other Slave. Several physical layer devices and drivers will allow you to force 1000 Mbit/s full duplex to eliminate autonegotiation issues. In this non-standard use, the designer must assure only one peer is configured as the clock master. Forcing duplex settings or turning off autonegotiation can become a permanent choice in a large installation, because the forced node is now non-compliant. When deployed, it will mean any future ethernet switch must be forced as well. A solution is to remove or fix the non-compliant nodes, rather than making more nodes non-compliant." - http://en.wikipedia.org/wiki/Gigabit_Ethernet

Just because you can do something doesn't mean it's how you're supposed to do it.

networker050184 · 05-02-2009, 12:31 AM

Did you read the last paragraph in the documented cited as a source?

rwwest7 · 05-02-2009, 01:09 AM

I read the entire thing. Which part are you asking about?

networker050184 · 05-02-2009, 12:59 PM

"This indicates that although operating speed is allowed to be manually selected by disabling Auto-Negotiation in Control Register 0, selecting 1000BASE-T mode of operation still requires that Auto-Negotiation be used. This can be accomplished by continuing to use Auto-Negotiation while limiting the advertising to 1000BASE-T capabilities."

rwwest7 · 05-03-2009, 11:48 PM

I get your point of how it's "possible" to force a port to 1000 Mb/Full. My point is that the standard calls for Auto only, and anything else is considered out of standards. Gigabit was designed to default to full duplex. And you can cause other problems by manually setting the port, I know it's burned into some peoples brains to manually set the speed/duplex on important ports and servers. But with gigabit you could actually cause problems doing that, problems that if you just leave it on Auto wouldn't occur.

rwwest7 · 05-03-2009, 11:49 PM

A little reading material for you:
Do Not Force Gigabit Ethernet Auto Negotiation to 1000/Full | Cisco Home Lab

APA · 05-04-2009, 06:55 AM

You can hard-code 1000 but you shouldnt...

It negotiates Master\slave but more importantly it first negotiates 100Mbps\Full then once that is successful it will go ahead via auto-negotiation and determines whether it can successfully operate at 1000Mbps\Full.

If 1000Mbps auto-ngeotiation fails it will realise this and operate at whatever it was successful negotiating before the attempt at 1000Mbps\Full -> either 100Mbps\Full or 100Mbps\Half.

networker050184 · 05-04-2009, 12:27 PM

I have read many articles and had many people tell me that you can't hard code gig without issues. I've hard coded many of them and they have ran flawless without one single issue.

You can keep letting your links negotiate and I'll hard code mine and see who runs into an issue first

My $.02

APA · 02:31 PM

Quote:

Originally Posted by networker050184

I have read many articles and had many people tell me that you can't hard code gig without issues. I've hard coded many of them and they have ran flawless without one single issue.

You can keep letting your links negotiate and I'll hard code mine and see who runs into an issue first

My $.02

Aww come on networker... it's not about who is right or who is wrong...

We are just exchanging ideas here.... I have come across many Gbps links being hardcoded... and they haven't run into issues yet.... but the recommended practice is to let Gigabit links autonegotiate as it's robust and works alot better then Fast Ethernet Auto negotiation ever did...

But you are right... each to their own

Hard-code or A-N... as long as the link is stable and works there shouldn't be a problem!

networker050184 · 05-04-2009, 02:43 PM

I'm not saying one way is wrong and one way is right. I'm saying there are two ways to do it. Maybe like rwwest7 stated its just from years of hard coding that has me suspicious of auto negotiation. In the wise words of Jeremy Ciora, if its auto you auto not use it (or something along those lines).